Believe it or not, while pigeons (and doves) may be birds, the adults who are raising young have a specially designed form of nourishing them that is parallel to (but nothing like!) “true” milks that mammals produce.
The digestive system consists, briefly, of the mouth (inside their bill, of course!), the esophagus, the crop (the center of today’s discussion), the proventriculus (the first of a two-part “stomach” region where some digestion takes place), the gizzard (the second stomach portion, where birds “chew”, because food is swallowed whole as the bills lack teeth), the intestines (supplied by the liver and pancreas), and all the way down to the cloaca, the common exit for wastes and reproductive products (sperm/egg). Interestingly enough, the forces of digestion can push the contents of the gizzard back to the proventriculus.
Now that we have mapped out the digestive system, let’s focus on the crop, where this mysterious milk is derived. This substance, having a curdled, rice-like appearance, is derived from the enlarged crop of a lactating pigeon (or similar bird, such as a dove, flamingo or male Emperor Penguin).
This secretion is rich in protein and also contains fat and more modest amounts of carbohydrates and minerals, among other nutrients.
Aren’t you glad you’re not a pigeon or similar bird? The milk sure looks quite gross to me. But hey, it’s all perspective.
(Source: http://people.eku.edu/ritchisong/birddigestion.html)
The Adventures of a Christian Bookworm 